It is proposed to carry out studies on the operation of the purine nucleotide cycle in various organs. The cycle will be studied in cell free extracts, in perfused organs, and in situ. The effects of specific inhibitors of enzymes of the purine nucleotide cycle will be investigated. The objective of the proposed experiments is to gain insight into the functions which have been proposed for the purine nucleotide cycle. The regulatory properties of the highly purified enzymes of the purine nucleotide cycle will be investigated with special emphasis on physiological concentration ranges of substrates, activators, and inhibitors. The control and function of 5'-nucleotidase will be studied with some emphasis on the enzyme from heart. This enzyme catalyzes the production of adenosine, which we believe to be an extracellular signal of the intracellular energy status. The hypothesis will be tested that 5-nucleotidase catalyzes a vectorial reaction in which intracellular AMP is expelled as extracellular adenosine.